Bend-capable stent prosthesis

ABSTRACT

Normally, when stents are bent, inside the body of the stented patient, there is head-to-head collision between facing V-points on the inside of the bend. However, by alternating between two whole numbers the number of struts between successive connectors around the circumference of each of the stenting rings, the V-points are caused to veer circumferentially in opposite directions as they approach each other on the inside of the bend, so allowing them to pass by each other without collision, thereby allowing in the same stent both close packing of the ring stack, and an enhanced ability to tolerate severe bending, after placement in the body.

PRIORITY

This application is a U.S. national stage application under 35 USC §371of International Application No. PCT/EP2007/054822, filed May 18, 2007,claiming priority to United Kingdom Patent Application No. 0609911.3,filed May 18, 2006, each of which is incorporated by reference in itsentirety into this application.

FIELD OF THE INVENTION

This invention relates to a stent prosthesis which is tubular and has amatrix of struts that provide a stenting action that holds bodily tissueradially away from any lumen defined by the stent matrix, around alongitudinal axis of the prosthesis. One such prosthesis is disclosed inapplicant's WO 01/32102.

BACKGROUND

Currently, the great majority of stents delivered transluminally andpercutaneously to a stenting site in a human body are made of abiologically compatible material which is a metal. Many stents are madeof stainless steel, and many others are made of nickel titanium shapememory alloy. The nickel titanium stents are invariably self-expandingstents that utilise a shape memory effect for moving between a radiallycompact transluminal delivery disposition and a radially larger stentingdisposition after placement in the body. Stainless steel stents areoften delivered on a balloon catheter, with inflation of the ballooncausing plastic deformation of the material of the struts, but otherstainless steel stents rely on the resilience of the steel to springopen when a surrounding sheath is retracted relative to the stent beingdeployed.

However, in all cases, it is difficult to endow the stent strut matrixwith a degree of flexibility that comes anywhere near the degree offlexibility of the natural bodily tissue at the stenting site. Thestrength and resilience of the stent matrix, that serves to pushradially outwardly the bodily tissue at the stenting site, is difficultto reconcile with the flexibility in bending that the natural tissuearound the stent is capable of exhibiting, in normal life of the patientcarrying the stent. It is one object of the present invention to improvethe performance of a stent prosthesis in bending, after it has beendeployed in the body of a patient.

To explain the problem, reference will now be made to applicant's WO01/32102, specifically drawing FIGS. 3 and 4, and the text, of WO01/32102. Indeed, accompanying drawing FIGS. 1 and 2 are the same asFIGS. 3 and 4 of WO 01/32102.

Looking at accompanying FIG. 1, we see part of the circumference of atubular workpiece of nickel titanium shape memory alloy, in side view.The tube has a diameter D and a multiplicity of slits 20, 22 and 24,through the wall thickness of the tubular workpiece, all parallel toeach other and to the longitudinal axis of the workpiece and creatingout of the original solid tubular workpiece a lattice which can beexpanded radially outwardly, (for example on a mandrel) to the expandedconfiguration of drawing FIG. 2 (again in side view). Out of themultitude of parallel slits can now be recognised as a sequence of 10stenting rings, all displaying a zig-zag advance around thecircumference of the prosthesis. Terminal zig-zag rings 30 are composedof 24 struts 32 interspersed by points of inflection 34, giving the endview of the prosthesis the appearance of a crown with twelve points.

The eight zig-zag rings at intermediate points along the length of thestent, between the two end rings 30, are referenced 36. They are made upof struts 38 which are all much the same length, somewhat shorter thanend struts 32. Between any two struts of any of the zig-zag stentingrings there is a point of inflection 40. In the two end rings 30, alltwelve of these points of inflection remote from the crown end of theterminal ring 30 are connected to a corresponding point of inflection40, head to head, in the next adjacent internal stenting ring 36.However, between any two internal stenting rings 36, not all the twelvepoints of inflection, found spaced around the circumference of theprosthesis, are joined to corresponding points of inflection on the nextadjacent stenting ring 36. Indeed, reverting to FIG. 1, it is easy tosee that there will be only four connector portions 42, linking any twoadjacent internal stenting rings 36.

Thinking about advance of the prosthesis of FIG. 1, in its compactdisposition, along a tortuous, transluminal, delivery path to thestenting site, as the stent bends around a sharp bend in the deliverypath, on the inside of any such bend, for example at point 44 on FIG. 1,the points of inflection facing each other across the gap 60 willapproach one another. Depending on the length of the diametricallyopposed connector portions 42 connecting stenting rings 36B and 36C, thetwo unconnected points of inflection will come into contact with eachother in the middle of the gap 60, in dependence upon how sharp is thebend that the stent is negotiating in the tortuous path at that time.The longer the axial gap between adjacent stenting rings, the greaterthe capability of the stent for negotiating ever tighter bends in thedelivery path lumen.

But what of the performance of the stent in bending, after it has beendeployed at the stenting site.

We can see from FIG. 2 that the pattern of connector portions 42 issymmetrical. That is to say, standing on one of these connectorportions, and looking along the length of the prosthesis, the pattern ofconnectors to the left of the line of view is a mirror image of thepattern of connectors to the right of that line of view. If we switch toconsideration of drawing FIG. 3, which shows a portion of the strutnetwork of the stent of FIGS. 1 and 2, this is more readily evident.Just as points of inflection on the inside of a tight bend of the stentin its compact disposition of FIG. 1 can butt up against each other faceto face, so can the same phenomenon occur when the expanded stent ofFIG. 2 is subject to sharp bending. Any such intermittent abutment ofotherwise free points of inflection is liable to have negative effectsincluding, for example, irritation or injury to bodily tissue caughtbetween the abutting points of inflection, or even incipient buckling ofthe stent with the potential to reduce flow of bodily fluid through thestent lumen to dangerously low levels.

It is one object of the present invention to mitigate these risks.

SUMMARY OF THE INVENTION

The matrix of struts of a radially expandable stent can be looked uponas a two dimensional lattice (when the tubular stent is opened out flaton a plane) and if the lattice has a regular structure (which itinvariably does) then it is possible to define the lattice using aconcept familiar in crystallography, namely, the “unit cell”characteristic of a space lattice of points, with each point of thespace lattice corresponding to one of the connector portions in thestent matrix. Conventionally, as in the structure shown in FIGS. 1 to 3discussed above, the unit cell is aligned with the longitudinal axis ofthe prosthesis. In accordance with one aspect of the present invention,however, the axially adjacent stenting rings are separated only by asmall gap, and the unit cell is deliberately “skewed” with respect tothe longitudinal axis of the prosthesis. This has the consequence that,when the expanded stent prosthesis is sharply bent, points of inflectionthat would otherwise approach each other head to head are prompted bythe stresses arising in the lattice of struts to shear sideways, inopposite directions around the circumference of the stent prosthesis sothat, when the tightness of the bend is finally such as to bring thepoints of inflection close to each other, they pass side by side ratherthan impact head to head.

Note that the axial gap between two radially expanded rings of astraight stent is virtually identical to the length of the gap betweenthe same two rings in the compressed stent n the delivery catheter. Butthe points of inflection are much further away from the longitudinalaxis, with the consequence that the amount of axial movement of facingpoints of inflection, for any particular degree of bending of the axis,is much greater with the stent radially expanded. A small axial gapmight therefore suffice, in the delivery disposition of a stent whilebeing inadequate to prevent head to head impact in the expandeddisposition.

The small gap between axially adjacent stenting rings is important forthe establishment of usefully high radially outwardly directed stentingforces. It is the tendency of the points of inflection (peaks) to passby each other, when the stent bends, in overlapping side-by-siderelationship, that opens up the possibility to keep the gap so small.

A relatively simple way to accomplish this desirable result is toarrange that, when the number of struts “N” of any stenting ring B lyingbetween any two adjacent connector portions is such that N/2 is an evennumber, so that the connector portions at one axial end of ring B cannotlie circumferentially halfway between any two connector portions on theother axial end of ring B. Note that in FIG. 2 above, there are sixstruts of any particular stenting ring 36 between adjacent connectorportions 42 on the same axial end of that stenting ring 36. Half of sixis three, and three is not an even number. Proceeding from anyparticular connector portion 42 of the matrix of FIG. 2, it takes threestruts to reach the next adjacent connector portion, whichever path onetakes when departing from the base connector portion 42. In accordancewith the present invention, the number of struts taken to reach the nextadjacent connector portion 42 is not always the same. In consequence,the stresses imposed on the struts by bending the prosthesis sharply(into a banana shape) are going to be distributed asymmetrically withrespect to any particular connecter portion 42 and it is this asymmetricstress distribution that will skew the free points of inflectionrelative to those facing them in the next adjacent stenting ring, sothat they do not abut each other head to head on the inside of the bendof the banana shape.

Thus, in accordance with another aspect of the invention, there isprovided a prosthesis that is expandable from a radially compactdelivery disposition to a radially expanded stenting disposition, and iscomposed of a stack of zig-zag stenting rings of struts that end inpoints of inflection spaced around the circumference of a stenting lumenthat is itself on a longitudinal axis of the stent, each of the pointsof inflection being located at one or the other of the two axial ends ofeach ring, with adjacent rings A, B, C in the stack being connected bystraight connectors linking selected facing pairs of points ofinflection of each two adjacent rings, circumferentially interveningpairs of facing points of inflection being unconnected, and withprogress from strut to strut via the points of inflection, around thefull circumference of one of the stenting rings B, namely one that islocated axially between adjacent rings A and C in the stack, theconnector ends encountered during such progress connect ring Balternately, first to ring A, then to ring C, then to ring A again, andso on characterised in that the connectors are parallel to thelongitudinal axis and are shorter than said strut length the pairs ofunconnected points of inflection remain facing, in the radially expandeddisposition, for as long as the longitudinal axis remains a straightline the number of struts in ring B that lie between successive saidconnector ends that join ring B alternately to ring A, then ring C, is awhole number that alternates between two different values; and theconnectors are so short that, when the stent functioning as a stent iscaused to bend, such that the longitudinal axis becomes arcuate, thefacing pairs of unconnected points of inflection that are on the insideof the bend eventually pass axially past each other, side by side,circumferentially spaced from each other, rather than impacting on eachother, head to head.

A stent construction in accordance with the invention is only marginallymore complex than the simple and “classic” zig-zag stenting ringconstruction evident from drawing FIGS. 1 to 3. The stenting rings canbe a simple zig-zag construction of struts all the same length, and theconnector portions can be nothing more than a plane of abutment betweenabutting points of inflection in adjacent zig-zag stenting rings, orsimple, short, straight portions aligned with the longitudinal axis ofthe prosthesis. This is advantageous, when it comes to modelling thefatigue performance of the stent, something of significant importancefor government regulatory authorities and for optimising stentperformance long term.

There is another valuable performance enhancement that the presentinvention can deliver, namely attainment of full performance of anyparticular “theoretical” stent matrix. In reality, every placement of astent is an individual unique event. To some extent, every stent ofshape memory alloy has had its remembered shape set in a unique heattreatment step. Referring back, once again, to WO 01/32102, we set theremembered shape before removing bridges of “scrap” material betweenstenting rings. In consequence, remembered shapes are highly orderly andregular, much closer to the “theoretical” zig-zag shape than can beattained when the rings are only connected by a minimum of connectorsduring the shape-setting step. We can have this advantage also withstents in accordance with the present invention, to optimise the bendingperformance of the stents, and the fatigue resistance that comes fromhaving stress distributions close to optimal, every time.

For a clear understanding of the invention definitions are useful for“strut length” and “connector length”. Fortunately, such definitions aremore or less self-evident, after consideration of how stents are made.

Normally, one begins with a tubular workpiece and creates in it amultitude of slits that extend through the wall thickness. They havetheir length direction more or less lengthwise along the tube.Circumferentially, adjacent slits are axially staggered. This is notunlike the way of making a simple “expanded metal” sheet havingdiamond-shaped apertures, familiar to structural engineers, and thosewho clad dangerous machinery in see-through metal sheet material toserve as safety guards.

For stent making, a useful extra step is to remove many of the residuallinks between adjacent diamonds. See again WO 01/32102, mentioned above.

The slit creation step can be by a chemical process such as etching or aphysical process such as laser cutting. For nickel titanium shape memoryalloys, the usual method is laser cutting.

So, now, how to define strut length and connector length? These lengthsemerge quite simply from an inspection of the axial lengths by whichcircumferentially adjacent slits overlap. For a strut length one wouldmeasure axially from the end of one slit (that is defining one of thetwo flanks surfaces of the strut under consideration) to the end of thecircumferentially next adjacent slit that has, as one of its defininglong walls, the other flank surface of the strut whose length is to beascertained. This method yields relatively short lengths. It is as ifone were a tailor, and were to measure arm length from the armpit ratherthan from a point on top of the shoulder of the person being fitted.

The same logic applies when determining connector lengths. Theycorrespond to the length of the gap that is created, when material ismoved from the stent workpiece, in the unslitted material between twoco-linear slits through the wall of the workpiece, said removal ofmaterial revealing two axially facing points of inflection when thestent matrix is subject to radial expansion. Thus, in the limiting case,the connector length is the same as the width of the laser beam thatremoved material to create that gap. Again, see WO 01/32102 mentionedabove.

Of course, connector lengths and strut lengths can vary over the stent.Some of its stenting rings may have longer struts than others. However,except for very special cases, a stent is indifferent to rotation aboutis long axis, so that changes in the rotational orientation of the stentrelative to the bodily lumen being stented, during advancement of thestent along the lumen to the stenting site, do not render the stentunfit for placement. Thus, for purposes of clarity in the here-claimedinvention, it will always be possible to divine clearly a strut lengthand a connector length, for testing whether the definition of theinvention is met in any particular zone of a stent that corresponds totwo adjacent stenting rings and the gap in between.

With the published state of the art there are disclosures, such as inUS2004/0117002 and US 2003/0225448, of stents composed of zig-zagstenting rings with straight connectors that join adjacent stentingrings peak-to-peak and with alternating whole numbers of struts lyingbetween circumferentially adjacent connectors terminating in any onering of such struts. Such stents exhibit face to face (otherwise herecalled “peak to peak”) facing points of inflection in the radiallycompact pre-expanded disposition of the stent. Such stents arerelatively easy to make by laser cutting of a precursor tube of rawmaterial. Whether such stents still exhibit face to face points ofinflection after expansion is unclear. What happens when the stents bendis also unclear. What is clear is that the writers of these priorpublications did not include any teaching about how facing points ofinflecting may tend to move in opposite circumferential directions onbending of the stent, and thereby ease away from head to head collision.A failure to recognise this phenomenon results in a failure toappreciate the scope to reduce the length of the connectors connectingadjacent stenting rings, thereby missing a chance to maximise radiallystenting force and strut coverage of the wall of the bodily lumen thathas been stented.

The disclosure of WO98/20810 is instructive. It describes laser cutstents of nickel titanium shape memory alloy, with zig-zag stentingrings that expand to a stenting diameter. It teaches that the straightconnectors linking axially adjacent stenting rings are to be at a slantto the longitudinal axis so that what would otherwise be the facingpoints of the “V-shaped segments” are circumferentially staggered, tominimise contact between these peaks when the expanded stent is bentsuch that the longitudinal axis becomes arcuate. Another reason forstaggering the V-points around the circumferences is to improve thehomogeneity of coverage of the lumen wall with the strut matrix of thestent, to leave no zones of coverage of the lumen wall tissue that aremore sparse than other zones. The connectors shown in the drawings doappear to be quite long and it is of course self-evident that, thelonger the connectors, the longer are the gaps between axially adjacentzig-zag rings, such gaps corresponding to sparse coverage of the lumenwall bodily tissue in the zones of tissue in the gaps between the rings.In other words, the shorter the connectors, the less need there is tostagger the V-point peaks circumferentially, in order to maintain lumenwall coverage by the matrix of struts of the stent.

When assimilating the disclosure value of WO98/20810 it is instructiveto imagine the stent in radially fully expanded disposition. Thecircumferential arc between two points of inflection is multiple timesmore than in the radially compressed delivery, and multiple times morethan the circumferential distance between the opposite ends of aslanting connector. This has the consequence that the degree by whichpeak to peak impact is alleviated, by a short slanting connector, isdisappointingly small, and gets relatively smaller with every increasein diameter of the expanded stent. By contrast, with the presentinvention, the greater the diameter of the expanded stent, the morepowerful the effect to circumferentially stagger the points ofinflection.

It will be evident to the skilled reader that the term “stenting ring”can be understood also to include in its scope successive turns around astent lumen of a spiral that is composed of struts in a zig-zagarrangement which spiral advances along the stent lumen away form one ofthe stent and towards the other.

Struts need not be of constant cross-section. Indeed, for optimisationof stress distribution within the struts, and hence of the fatigueperformance of the stent the cross-section will indeed change, along thelength of each strut. The struts need not all be the same as each other.There could be different strut species, either from ring to ring or,indeed, within a stenting ring. A common arrangement is to have rings oflonger struts at each end of the stent, the shorter struts at amid-length portion, providing greater radially outward stenting force.

The stent can be a bare stent or a covered stent such as a stent graft.The stent may be a drug-eluting stent. The stent may have a functionother than to hold a bodily lumen open against stenosis. For example,the stent could be part of a filter device for temporary placement in abodily lumen, or an anchor for some other device that is to perform atherapeutic function within a bodily lumen.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, and to show moreclearly how the same may be carried into effect, reference will now bemade, by way of example, to the accompanying drawings, in which:

FIGS. 1 and 2 are side views of the stent described in WO 01/32102, withFIG. 1 in the compact delivery disposition and FIG. 2 in the radiallyexpanded deployed disposition of the prosthesis.

FIG. 3 is a diagram of symmetrical matrix of connector portions (notunlike the embodiment of FIGS. 1 and 2), opened out flat on a plane, and

FIG. 4 is a diagram corresponding to that of FIG. 3, but with a matrixof struts and connectors in accordance with the present invention

FIG. 5 is a photographic side view of a stent prosthesis which exhibitsthe strut and connector matrix of FIG. 4, expanded but not subject toany bending stresses; and

FIG. 6 is a photographic side view of part of the stent prosthesis ofFIG. 5, but bent into a “banana” shape to reveal how the points ofinflection move relative to each other and relative to the addressedunbent configuration of FIG. 5.

DETAILED DESCRIPTION

What is shown in FIGS. 1, 2 has been described above and in applicant'searlier WO 01/32102. The reader is referred to the passages above and tothe prior publication.

FIG. 3 is not unlike the embodiment of FIGS. 1 and 2, but the length ofthe elongate connectors 42 helps to reveal the pattern of connectors inthe lattice.

FIG. 4 reveals a matrix of struts 38 and connectors 42 spacing apart asuccession of zig-zag stenting rings 36 (four are visible in FIG. 4).Starting from connector 42A, we can reach adjacent connector 42B via asequence of three struts 38ABC. But not all adjacent connectors are asclose. Consider adjacent connector 42C. It takes five struts, namelystruts 38D to H, to reach connector 42C. The pattern is repeatedthroughout the matrix. Note that the connector 42D that links zig-zagrings 36C and 36D is displaced circumferentially sideways from connector42A, unlike the arrangement in FIG. 3. If we imagine in FIG. 4connectors 42A and 42D lying on the inside of a severe bend of theexpanded stent matrix, so that the points of inflection 40X on zig-zagring 36B, and the points of inflection 40Y on zig-zag ring 36C, aremoving towards each other, the stresses imposed by connector 42A onstenting ring 36B and those imposed on stenting ring 36C by connector42D will be unsymmetrical. It does not require a great exercise ofimagination to visualise points of inflection 40× and points ofinflection 40Y failing to meet each other face to face when the bend istight enough but, instead, sliding past each other, with spacing.

Turning to drawing FIGS. 5 and 6, we see occurring in practice exactlywhat one can, with a degree of imagination, visualise occurring from thediagram of FIG. 4. Whereas the free points of inflection in FIG. 5, theunstressed configuration of the expanded stent, are bravely facing eachother without any circumferential staggering, as soon as the prosthesisis subject to external stresses that bend it into the banana shapeevident from FIG. 6, what was previously and orderly face to faceconfiguration of points of inflection has now become a staggeredconfiguration, not just on the exact inside of the bend but also on theflanks of the bend that are facing the viewer in the side view of FIG.6.

Self-evidently, the construction of FIG. 5 is hardly more complex thanthat of FIG. 2. Likewise, the construction of FIG. 4 is self-evidentlyhardly more complicated than the FIG. 3 matrix. It is one advantage ofthe present invention that the useful result evident in FIG. 6 can beachieved with a lattice that is barely more complicated than that of theclassic lattice of WO 01/32102. That is of course not to say that thebenefits of the invention are not achievable with more complicatedconstructions. There is now an enormous multitude of stent latticepossibilities and those who are promulgating relatively complicatedlattices would doubtless assert that their specific constructions bringuseful benefits. Doubtless the simple principle of the present inventioncan be incorporated into these more complicated arrangements, as skilledand experienced stent design readers will appreciate.

As increasing sophistication of design of stents allows them to performin ever more demanding locations in the body, the need for stentflexibility in bending continues to increase. for maximum flexibility,one would wish for a minimum of connector portions between stentingrings. However, the point about connectors is that they do serve to keepapart from each other portions of stenting rings that might otherwisecollide. There is therefore a tension between the objective ofpreventing collisions and the objective of greater flexibility. Thepresent invention aims to make a contribution to this delicatecontradiction, by using just a few connectors to encourage approachingpoints of inflection to, as it were, politely step to one side, inopposite directions, as they approach each other, rather thanconfronting each other head to head. Given the strength that effectivestents need to exhibit, to keep bodily tissue displaced radiallyoutwardly from the bodily lumen being stented, there should be enoughstrength in even just a few connectors to ease the points of inflectionpast each other, because only a relatively small “push” on the points ofinflection, in circumferentially opposite directions, should be enoughto prevent a peak-to-peak confrontation. Otherwise, when the stent inthe body is not called upon to bend, then the connectors do not have togo to work to urge the facing points of inflection to move in oppositecircumferential directions. The stresses in the stent matrix are thosethat arise anyway, when the surrounding tissue is urging the stentmatrix to bend from a straight tube to a banana shape. Accordingly, thestresses within the stent matrix are in harmony with the stresses thatthe surrounding body tissue is experiencing, and imposing on the stent.This harmony should be of assistance in matching the performance of themetal stent matrix to the resilient properties of the surrounding bodilytissue.

There is no requirement that the skewed arrangement, that the presentinvention proposes, be reproduced throughout the stent lattice. Forexample, it may be desirable to make one portion of a stent morebend-capable than other parts. In such a case, it may be useful toconfine the skewed connector distribution to those parts of the stentwhich are to be relatively more bend-capable. It hardly needs to beobserved that the bend capability of a stent portion, before it beginsto buckle, should be high enough to incur the risk of abutment ofapproaching points of inflection in adjacent stenting rings, to makeincorporation of the skewed distribution of the invention worthwhile.Generally, the sparser the population of connector portions between thepopulation of connector portions between stenting rings, the morebend-capability will be available.

FIG. 3 shows 6 struts between adjacent connectors in the same circle,and FIG. 4 shows 8. With 10 connectors, an unsymmetrical arrangement ofthe present invention suggests a heavily skewed split of 3/7 in thenumber of struts between each connector and the nearest one in theaxially next adjacent ring of connectors (with the symmetricalarrangement being 5/5). 12 connectors seem scarcely more attractivebecause then the split is 4/8, still somewhat heavily skewed relative toa symmetrical 6/6 split of struts between connectors, but 14 connectorsseems more attractive because that permits a 6/8 split which is close tothe symmetrical 7/7 split of a symmetrical arrangement. One seeks anarrangement that is skewed enough to urge the approaching points ofinflection on the inside of the bend to pass each other elegantly, butnot such a pronounced skew that stresses in the stent lattice showpronounced differences, depending where in the lattice one is measuringthem.

Generally, there will be up to 6 connectors in each circle ofconnectors. 3 or 4 connectors per ring are presently favoured but thenumber of connectors falls to be determined in harmony with many otherdesign aspects of the stent lattice, as stent designers well know.

The radially outwardly directed force that a stent can exert against thebodily tissue forming the walls of the stented bodily lumen willinevitably be somewhat reduced, with increasing length of the gapsbetween axially adjacent stenting rings of the stent. Clearly then, onewould choose short connectors to maximise stenting radial force. In ahigh flex location for the stent measures must be taken, to preventcollisions between adjacent stenting rings when the stent is subjectedto serve bending. A particularly useful technical effect of the presentinvention is that the short connector portions allow close proximity ofaxially adjacent stenting rings (and so a high stenting force) yet nocollisions between the closely adjacent rings when the stent sufferssevere bending.

EXAMPLE

To assist readers to grasp the physical dimensions of stents that arepreferred embodiments of the present invention, we set out in the Tablebelow some representative dimensions for stents studied by theApplicant. It is to be understood that these dimensions are provided notto signify precise dimensions that work better than others but merelydimensions within the ranges here contemplated.

TABLE Each zig-zag ring Connector Strut Strut extended Number of widthlength Connector length Product struts (μm) (mm) (mm) length (mm)* A 24160 1.95 0.8 1.4 B 36 100 1.45 0.5 1.0 C 30 100 1.45 0.5 1.0 D 32 1351.55 0.5 1.0 *This is the full length that lies between the ends of twoco-linear slits axially spaced from each other that create the twoaxially-facing V-points of inflection of two adjacent zig-zag rings

One message to be taken from the Table is that strut lengths are goingto be, in general, significantly more than 1 mm while connectors aregoing to exhibit a length significantly below 1 mm. The points ofinflection, in themselves, typically have an axial length of 0.25 mm or0.30 mm, which is typically around two or three times the width (in thecircumferential direction) of one of the struts. Thinking of a point ofinflection as a zone where the material of two struts comes together inan unslitted block of material, that block will have the width of twostruts and an axial length that is similar to, or a bit longer than,such width.

In general, connectors lengths will be 0.8 mm or less, likely 0.6 mm orless. Strut lengths will likely be more than 1.25 mm, likely is a rangeof from 1.3 to 2.2 mm or more specifically 1.4 to 2.0 mm. One favouredconstruction has 32 struts per ring, such as in Product D in the Table.

For the sake of clarity, and the avoidance of doubt, the “points ofinflection” referred to in this specification are not a reference to thepoint of inflection that each strut exhibits, mid-way along its length,which more or less inevitably appears when the slitted stent precursortube is radially expanded form its original diameter to its workingstenting diameter.

The invention claimed is:
 1. A prosthesis that is expandable from aradially compact delivery disposition to a radially expanded stentingdisposition, comprising: a stack of zig-zag stenting rings of strutsthat end in points of inflection spaced around the circumference of astenting lumen that is itself on a longitudinal axis of the stent, eachof the points of inflection being located at one or the other of the twoaxial ends of each ring, with adjacent rings A, B, C in the stack beingconnected by straight connectors linking selected facing pairs of pointsof inflection of each two adjacent rings, circumferentially interveningpairs of facing points of inflection being unconnected, and withprogress from strut to strut via the points of inflection, around thefull circumference of one of the stenting rings B, namely one that islocated axially between adjacent rings A and C in the stack, theconnector ends encountered during such progress connect ring Balternately, first to ring A, then to ring C, then to ring A again, andso on, wherein: the connectors in rings A, B, and C are parallel to thelongitudinal axis and have a length which is less than 1 mm; the strutsin rings A, B, and C have the same cross-section and a strut length ofmore than 1.25 mm, wherein the number of struts in ring B that liebetween successive said connector ends that join ring B alternately toring A, then ring C, is a whole number that alternates between twodifferent values; the pairs of unconnected points of inflection remainfacing, in the radially expanded disposition, for as long as thelongitudinal axis remains a straight line; and when the stent is causedto bend such that the longitudinal axis becomes arcuate, the facingpairs of unconnected points of inflection that are on the inside of thebend eventually pass axially past each other, side by side,circumferentially spaced from each other, rather than impacting on eachother, head to head.
 2. A stent that is expandable from a radiallycompact delivery disposition to a radially expanded stentingdisposition, comprising: a plurality of stenting rings, each stentingring including a plurality of struts with adjacent struts connecting inpoints of inflection, the points of inflection of adjacent stentingrings facing each other along an axis parallel to a longitudinal axis ofthe stent, adjacent stenting rings A, B, and C being connected bystraight connectors parallel to the longitudinal axis of the stent, theconnectors linking selected facing pairs of points of inflection ofadjacent stenting rings where stenting ring B is connected to stentingrings A and C in an alternating pattern about a circumference of thestent, each of the connectors having a length less than 1 mm, each ofthe struts having the same cross-section and a length greater than 1.25mm, wherein the facing points of inflection of adjacent stenting ringsaxially pass by each other when the stent is bent and the longitudinalaxis becomes arcuate.
 3. The stent according to claim 2, wherein thenumber of struts “N” of stenting ring B lying between any two adjacentA-B connectors is such that N/2 is an even number.
 4. The stentaccording to claim 2, wherein the number of struts “N” of stenting ringB lying between any two adjacent A-B connectors is such that N/2 is anodd number and N is more than
 10. 5. The stent according to claim 2,wherein lattice points of the connectors together exhibit a helical paththat is coaxial with the longitudinal axis of the prosthesis.
 6. Thestent according to claim 2, wherein each of the stenting rings iscomposed of struts that have all the same length, such that each of thepoints of inflection in such a ring lie in one of two circles transverseto the longitudinal axis of the prosthesis.
 7. The stent according toclaim 2, comprising a shape memory alloy.
 8. The stent according toclaim 2, which undergoes plastic deformation upon expansion to itsstenting disposition.
 9. The stent according to claim 2, wherein theconnector length is not more than 0.8 mm.
 10. The stent according toclaim 2, wherein the connector length is less than 0.6 mm.
 11. The stentaccording to claim 2, wherein the strut length is in a range of from 1.3mm to 2.2 mm.
 12. The stent according to claim 2, wherein the strutlength is in a range of from 1.4 mm to 2.0 mm.
 13. The stent accordingto claim 2, wherein each of the stenting rings A, B, and C includes 32struts.
 14. The stent according to claim 2, wherein four evenly spacedconnectors connect stenting ring B to stenting ring A.